1. Field of the Invention
The invention pertains to a xe2x80x9cleads over chipxe2x80x9d (LOC) semiconductor die assembly and, more particularly, to a method and apparatus for reducing the stress resulting from lodging of filler particles present in plastic encapsulants between the undersides of the lead frame leads and the active surface of the die and improving lead locking of the leads in position over a portion of the active surface of a semiconductor die assembly.
2. State of the Art
The use of LOC semiconductor die assemblies has become relatively common in the industry in recent years. This style or configuration of semiconductor device replaces a xe2x80x9ctraditionalxe2x80x9d lead frame with a central, integral support (commonly called a die-attach tab, paddle, or island) to which the back surface of a semiconductor die is secured, with a lead frame arrangement wherein the dedicated die-attach support is eliminated and at least some of the leads extend over the active surface of the die. The die is then adhered to the lead extensions with an adhesive dielectric layer of some sort disposed between the undersides of the lead extensions and the die. Early examples of LOC assemblies are illustrated in U.S. Pat. No. 4,862,245 to Pashby et al. and U.S. Pat. No. 4,984,059 to Kubota et al. More recent examples of the implementation of LOC technology are disclosed in U.S. Pat. Nos. 5,184,208; 5,252,853; 5,286,679; 5,304,842; and 5,461,255. In instances known to the inventors, LOC assemblies employ large quantities or horizontal cross-sectional areas of adhesive to enhance physical support of the die for handling.
Traditional lead frame die assemblies using a die-attach tab place the inner ends of the lead frame leads in close lateral proximity to the periphery of the active die surface where the bond pads are located, wire bonds then being formed between the lead ends and the bond pads. LOC die assemblies, by their extension of inner lead ends over the die, permit physical support of the die from the leads themselves, permit more diverse (including centralized) placement of the bond pads on the active surface, and permit the use of the leads for heat transfer from the die. However, use of LOC die assemblies in combination with plastic packaging of the LOC die assembly has demonstrated some shortcomings of LOC technology as presently practiced in the art.
One of the short comings of the prior art LOC semiconductor die assemblies is that the tape used to bond to the lead fingers of the lead frame does not adequately lock the lead fingers in position for the wire bonding process. At times, the adhesive on the tape is not strong enough to fix or lock the lead fingers in position for wire bonding as the lead fingers pull away from the tape before wire bonding. Alternatively, the lead fingers will pull away from the tape after wire bonding of the semiconductor die but before encapsulation of the semiconductor die and frame, either causing shorts between adjacent wire bonds or the wire bonds to pull loose from either the bond pads of the die or lead fingers of the frame.
After wire bonding the semiconductor die to the lead fingers of the lead frame forming an assembly, the most common manner of forming a plastic package about a die assembly is molding and, more specifically, transfer molding. In this process (and with specific reference to LOC die assemblies), a semiconductor die is suspended by its active surface from the underside of inner lead extensions of a lead frame (typically Cu or Alloy 42) by a tape, screen print or spin-on dielectric adhesive layer. The bond pads of the die and the inner lead ends of the frame are then electrically connected by wire bonds (typically Au, although Al and other metal alloy wires have also been employed) by means known in the art. The resulting LOC die assembly, which may comprise the framework of a dual-in-line package (DIP), zig-zag in-line package (ZIP), small outline j-lead package (SOJ), quad flat pack (QFP), plastic leaded chip carrier (PLCC), surface mount device (SMD) or other plastic package configuration known in the art, is placed in a mold cavity and encapsulated in a thermosetting polymer which, when heated, reacts irreversibly to form a highly cross-linked matrix no longer capable of being re-melted.
The thermosetting polymer generally is comprised of three major components: an epoxy resin, a hardener (including accelerators), and a filler material. Other additives such as flame retardants, mold release agents and colorants are also employed in relatively small amounts.
While many variations of the three major components are known in the art, the focus of the present invention resides on the filler materials employed and their effects on the active die surface and improved lead locking of the lead fingers of the frame.
Filler materials are usually a form of fused silica, although other materials such as calcium carbonates, calcium silicates, talc, mica and clays have been employed for less rigorous applications. Powdered, fused quartz is currently the primary filler used in encapsulants. Fillers provide a number of advantages in comparison to unfilled encapsulants. For example, fillers reinforce the polymer and thus provide additional package strength, enhance thermal conductivity of the package, provide enhanced resistance to thermal shock, and greatly reduce the cost of the polymer in comparison to its unfilled state. Fillers also beneficially reduce the coefficient of thermal expansion (CTE) of the composite material by about fifty percent in comparison to the unfilled polymer, resulting in a CTE much closer to that of the silicon or gallium arsenide die. Filler materials, however, also present some recognized disadvantages, including increasing the stiffness of the plastic package, as well as the moisture permeability of the package.
Another previously unrecognized disadvantage discovered by the inventors herein is the damage to the active die surface resulting from encapsulant filler particles becoming lodged or wedged between the underside of the lead extensions and the active die surface during transfer molding of the plastic package about the die and the inner lead ends of the LOC die assembly. The filler particles, which may literally be jammed in position due to deleterious polymer flow patterns and flow imbalances in the mold cavity during encapsulation, place the active die surface under residual stress at the points of contact of the particles. The particles may then damage the die surface or conductive elements thereon or immediately thereunder when the package is further stressed (mechanically, thermally, electrically) during post-encapsulation handling and testing.
While it is possible to employ a lower volume of filler in the encapsulating polymer to reduce potential for filler particle lodging or wedging, a drastic reduction in filler volume raises costs of the polymer to unacceptable levels. Currently available filler technology also imposes certain limitations as to practical, beneficial reductions in particle size (currently in the 75 to 125 micron range, with the larger end of the range being easier to achieve with consistency) and in the shape of the filler particles. While it is desirable that particles be of generally spherical shape, it has thus far proven impossible to eliminate non-spherical flakes or chips which, in the wrong orientation, maximize stress on the die surface.
Ongoing advances in design and manufacturing technology provide increasingly thinner conductive, semiconductive and dielectric layers in state-of-the-art dice, and the width and pitch of conductors serving various purposes on the active surface of the die are likewise being continually reduced. The resulting die structures, while robust and reliable for their intended uses, must nonetheless become more stress-susceptible due to the minimal strength provided by the minute widths, depths and spacings of their constituent elements. The integrity of active surface die coats such as silicon dioxide, doped silicon dioxides such as phosphorous silicate glass (PSG) or borophosphorous silicate glass (BPSG), or silicon nitride, may thus be compromised by point stresses applied by filler particles, the result being unanticipated shortening of device life if not immediate, detectable damage or alteration of performance characteristics.
The aforementioned U.S. Pat. No. 4,984,059 to Kubota et al. does incidentally disclose several exemplary LOC arrangements which appear to greatly space the leads over the chip or which do not appear to provide significant areas for filler particle lodging. However, such structures may create fabrication and lead spacing and positioning difficulties.
In addition to solving the problems associated with filler particle lodging and damage, it is desirable to have improved lead locking of the lead fingers of the frame during operations involving the semiconductor die. If the lead fingers have increased flexibility, the adhesive used to bond the lead frame to the semiconductor die is more effective in locking the lead fingers in position. Previously, improving lead finger locking has been approached from the perspective of improved adhesives, rather than increasing the flexibility of the lead fingers.
From the foregoing, the prior art has neither provided for improved locking of the lead fingers to the semiconductor die, nor recognized the stress phenomenon attendant to transfer molding and the use of filled encapsulants, nor provided an LOC structure which beneficially accommodates this phenomenon.
The present invention provides a lead-supported die assembly for an LOC arrangement that substantially reduces the stress that may otherwise potentially form between the leads and the active die surface due to the presence of filler particles of the polymer encapsulant and improved lead locking of the leads in position over a portion of the active surface of a semiconductor die assembly. Accordingly, each lead of the lead frame between the bonding location of the die and the edge of the die is formed with a stress relief portion therein. The resulting enlarged volume of space between the leads and the active die surface will beneficially accommodate an increased amount of the underlying filler particle or particles of the polymer encapsulant. Accordingly, a stacking of filler particles in which the filler particles try to force the lead away from the die, thus causing stress in the connection between the lead and the die, is less likely to occur. Moreover, this stress relief portion allows flexibility in bending and torsion in the leads due to stress created during the transfer molding process as well as other processes. The resulting lead flexure in response to the filler material will produce an immediate reduction in the residual stress experienced by the active die surface. This lessened residual stress is carried forward in the encapsulated package after cure, permitting the package to better withstand the stresses of post-encapsulation handling and testing, including the elevated potentials and temperatures experienced during burn-in, without adverse effects. The resulting lead flexure also allows improved lead finger locking to the tape as less force is transferred to the tape from the flexure of the lead fingers, which force may cause the lead fingers to become dislodged therefrom prior to the wire bonding operations or, subsequently, during encapsulation of the assembly.
The LOC apparatus of the present invention comprises a lead frame to which the active surface of a die is adhered by an LOC tape, permitting the lead frame to physically support the die during pre-encapsulation handling and processing such as wire bonding. The free ends of the leads have a recessed portion formed therein extending over a longitudinal length of the lead end proximate the active surface of the die. With such an arrangement, intrusion of filler particles between the inner lead ends and the active surface of the die during the encapsulation process is beneficially accommodated.
Stated in more specific terms and on the scale of an individual lead and the underlying active surface of the die, proximate the dielectric adhesive (such as LOC tape, screen print or spin-on, as known in the art) disposed on the underside of a lead, the lead is minimized in cross-sectional area along the lead axis. The design permits an increased amount of filler particles to accumulate in this recessed area such that the filler is less likely to force the lead away from the die. Similarly, the lead may incorporate an arched or bowed portion proximate its bonding location to the die to accommodate an increased amount of filler particles between the lead and the active surface of the die. This design also permits the free end of the lead to flex in bending and torsion about this arched or bowed portion, so as to bend or twist as required while being retained in position by the tape or in the presence of a filler particle or particles lodged between that lead and the die.
It is contemplated that this flexibility or stress reduction portion or filler particle accommodation portion of the lead frame may be formed in several manners. For example, a certain amount of each lead finger where flexibility, stress reduction or filler particle accommodation is desired may have a portion of its thickness chemically etched away. Likewise, various erosion, electron beam, machining or other processes known in the art may be utilized to reduce the thickness of the lead finger at the desired location. The reduced thickness portion of the lead finger for flexibility, stress reduction and filler particle accommodation may also be created by deformation, coining, stamping or other such methods known in the art to thin material.